I found an interesting fact about plant protein in a botany textbook called "Introductory Plant Biology 11th edition" which I am using as a source of information for my upcoming gardening project.
Some plant food-storage organs, such as potato tubers and onion bulbs, store small amounts of proteins in addition to large amounts of carbohydrates. Seeds, in particular, however, usually contain proportionately larger amounts of proteins in addition to their complement of carbohydrates and are very important nutrition for humans and animals. One example of an important protein source in human and animal diets is wheat gluten (to which, incidentally, some humans become allergic). The gluten consists of a complex of more than a dozen different proteins.
A seed's proteins get used during germination and its subsequent development into a seedling.